i'm still waiting for the brown note to occur.
 
i don't want to be there,
i just want to hear about it.
 

The brown note actually occurred once back in 1999-2000.   It was at a Bad Manners gig..  They play a ska cover of "Black Night", which is actually very good if it gets past the intro...
 
A big roll around the toms, everyone hits the first note dead on cue, along with every house light...      Blackout!!  Silence...    Brown note from behind FOH desk... 

So what I'm hearing is an affirmation of what I'd always assumed: it's about acoustics #1, FOH talent #2 and gear #3.
 
IOW, you can't buy your way out of the dilemma, just like you can't buy your way to quality in the studio.

batsbrew
 
there was always a 'room of doom' that had such terrible acoustics, that nothing could be done,.
 
 
i say, if the PA sound bad, it starts with the soundman,
sits in the middle with the quality of the gear,
and ends with the room acoustics.
 
 
2 of those you can fix.
 

I think Batsbrew has it spot on here. I am lucky enough to live near a venue called The Stables Theatre in a village called Wavendon just outside the city of Milton Keynes in the UK. It is run by the singer Cleo Laine and was managed by her and her late husband the musician Johnny Dankworth. The main room is a 1500 seater auditorium called the Jim Marshall Theatre which has the perfect combination of great equipment, sound engineers and most importantly, was designed and built totally for acoustic perfection. I have been to many many shows here over the years, from solo singer / guitar to full rock performances at thundering volume. In fact, the last two gigs I have attended there cover both those extremes - Nick Lowe, and the classic 70's rock inspired prog rock band, Mostly Autumn.
 
The sound is never less than stellar, world class artists love to perform here, even putting it on their tour itinerary when they are selling out far larger venues on the same tour, and mainly because of the acoustics.
 
So, to answer your question Bit, I believe the venue has to be sonically designed for the task, which is a rare thing, and therefore PA sound becomes a hit and miss affair...
 
EDIT: For those interested in the UK - http://www.stables.org/ - If you plan to see an artist or band on a UK tour, it is well worth the extra mileage to book here - but you have to get in early...

Reasonable volume levels are a factor too.  I've been to shows where the object seemed to be lets see if we can rip the cones from the baskets...
 
The FOH talent.... stage talent..... and equipment are important and in the proper order I believe.   All 3 coupled with reasonable volume for the venue/room go into the quality the audience hears.

I saw Nine Inch Nails a few years ago in a notoriously bad acoustic environment.
 
The sound was close to impeccable. As loud as it was, it was surprising clear and well-defined. I even thought it was a bit sterile, if anything.
 
That's the only concert I've seen which managed to be both loud and that clean. I usually don't like concerts all that much because clarity is the first thing to be sacrificed. It's just too loud.

Maybe this is why I've never been that keen on gigs. Pretty sure my tinnitus originated from the metal gigs I attended in the 80's - Metallica, Dio, Helloween, Onslaught, Slayer etc. I'd be half deaf for the rest of the evening and some of the next day too. I don't ever recall thinking "this sound quality is great!" I just remember a wall of noise and wishing they'd turn it down a tad.
 
Some years ago I went to see a Zappa tribute band at a small venue. The musicians were stunning virtuosos and what they were playing was incredible, but the PA was so loud and distorted that I walked out after 20 minutes. I really think that a lot of people have cloth ears and don't even care.
 
It really makes you appreciate the sound of unamplified musicians.

There's no excuse for distortion in the PA.  That's just a sign of an inexperienced FOH eng whose gear is not up to the job.
 

Herb's spot on with the volume comment. There is going to be an ideal volume range for a given system/room/audience/genre. Exceed it at your own risk. This is something I've found to be consistently true over the 50 years I've been performing, that sometimes that final tweak you need is just lowering the mains by 6dB.
 
I attended a concert at NAMM years ago, in a hotel ballroom. It was sponsored by a PA manufacturer, who I guess assumed that the best advertisement for the gear was to show how loud it could get. Imagine stadium volume in a 25,000 square foot room with an audience of perhaps 300 people. The musicianship was stellar, but the sheer amount of air moving was unbearable. I lasted about 20 minutes, which was 19 minutes longer than I should have subjected my ears to it.
 
The odd thing was that most of the audience didn't seem to notice or mind, including the Cakewalk staff who were recording the performance. I'd love to hear that recording, just to hear what the concert actually sounded like. 

When I started doing FOH for our annual bike club rally one of my jobs was to collate info beforehand regarding noise levels for submission to the local council as part of our entertainment license.
 
It seems there are accepted levels for various events that impact on audience enjoyment.
for Live performances from rock/pop bands your looking in the region of 98 - 103 dbA in the audience.
Much below that and you loose that "live something" that sets apart a live performance from simply playing back a recording, even if it's a live recording.
 
of course, much above that and the audience starts backing away....  Which is where the rear fills kick in and push them back to the front again